The notion of RSVP really doesn't work any more in this country.
Years ago, you got a written invitation to a function - you wrote back, because that was the courtesy of the day.
Now, people either don't write back because they are coming, so they think they don't need to bother, and they turn up, or they don't write back because they are not coming, and they don't turn up.
The end result is, the hosts have no idea how may people to cater for.
For a sit-down wedding, this is more of an issue, but if people don't come, that's the way it goes.
If you value someone's friendship enough to invite them to share your special day, why would you ruin that friendship by turning the event into a financial transaction? The guilt is on the missing guest(s), the magnanimous acceptance is on the hosts.
I would not ask for money, more because of what it says about my lack of manners, than what it says about the guest who did not appear or notify me in advance..